The present invention relates to a communication method in digital radio communication suitable for application to a radio communication system such as a radio telephone system using, for example, a cellular scheme, and to a transmitter and a receiver to which the communication method is applied.
As a conventional communication scheme for sharing a wide frequency band among a plurality of users and conducting communication efficiently, such as a radio telephone system, there is, for example, the DS-CDMA (Direct Sequence-Code Division Multiple Access) scheme. In this DS-CDMA scheme, a transmission signal sequence is spread (multiplied) by a code to generate a wide band signal, and this signal is transmitted. Furthermore, on a receiving side, a received signal is multiplied by the same spreading code as that of the transmission side to yield an effect called despreading. Only a desired signal component is thus extracted out of the received signal.
FIG. 1 shows a transmission configuration in a cellular radio communication system to which the conventional DS-CDMA scheme has been applied. An information bit stream obtained at an input terminal 1 is subjected to processing such as coding and interleaving in a coding unit 2, then supplied to a multiplier 3, and multiplied and spread therein by a code intended for channel assignment obtained at a terminal 3a. The spread bit stream is randomized in a multiplier 4 of a subsequent stage by a long code obtained at a terminal 4a, and thereafter mapped in a symbol mapping unit 5 to a transmission symbol. As for this mapping method, there are various techniques depending on the communication scheme.
The transmission signal mapped in the symbol mapping unit 5 is multiplexed in an adder 6 with transmission signals of other systems as occasion demands, supplied to a transmission processing unit 7, subjected to high frequency processing such as modulation therein, thereafter converted in frequency to a frequency band to be radio-transmitted, and radio-transmitted from an antenna 8.
Assuming now that the information bit stream obtained at the input terminal 1 is, for example, 8 kbps, it is encoded in the coding unit 2 with a coding rate 1/2. The bit rate of the coded bit becomes 16 kbps. If the coded bit is spread with a spreading factor 64 in the multiplier 3, a bit stream of 1024 kcps (where cps represents Chip Per Second) is obtained. If the information bit stream is different in bit rate, the bit rate of the transmission signal can be made constant by changing the spreading factor to be used in the multiplier 3.
Furthermore, as for other transmission systems added in the adder 6 as well, various information bit streams can be mixed as information bit streams supplied to the coding units 2 of respective transmission systems, provided that the bit stream of the transmission signal supplied to the adder 6 is constant.
A configuration for receiving the signal subjected to the transmission processing using the conventional DS-CDMA scheme will now be described by referring to FIG. 2. A signal of a predetermined frequency band received by an antenna 11 is subjected to frequency conversion in a reception processing unit 12 to form an intermediate frequency signal. The received signal thus subjected to the frequency conversion is demodulated to yield a baseband symbol sequence. Out of this symbol sequence, a received bit stream is extracted in a bit extraction unit 13. The received bit stream thus extracted is supplied to a multiplier 14, and multiplied and descrambled by a long code obtained at a terminal 14a. The multiplied output of the multiplier 14 is supplied to a multiplier 15, and multiplied by a despreading code obtained at a terminal 15a. Despreading processing is thus conducted, and a coded bit stream is obtained. The coded bit stream is decoded in a decoding unit 16, and an information bit stream is obtained at a terminal 17.
If a signal of the case where the above described information bit stream of 8 kbps is transmitted as the bit stream of 1024 kcps is received by the configuration of FIG. 2, then the signal is despread in the multiplier 15 with a despreading factor 64 and an information bit stream of 8 kbps is obtained. Furthermore, if the despreading factor of the despreading code obtained at the terminal 15a is changed, then an information bit stream of a different bit rate can be coped with.
In the description given so far, the case where information bit streams of a plurality of bit rates are mixed and radio-transmitted by using the DS-CDMA scheme has been described. Also in the case where they are radio-transmitted by using the TDMA (Time Division Multiple Access) scheme, however, it is also possible to mix information bit streams of a plurality of bit rates. FIG. 3 is a diagram showing the structure of one frame in the case of 8-TDMA structure in which one frame is formed of 8 time slots ranging from slot 1 to slot 8.
Assuming now slots to be assigned in the case where the transmission rate per slot is 8 kbps, for example, slots 1 and 2 are respectively assigned to users A and B having a transmission rate of 8 kbps and communication of the transmission rate of 8 kbps is conducted by using the slot 1 or 2. Furthermore, two slots composed of slot 3 and slot 4 are assigned to a user C having a transmission rate of 16 kbps and communication of 16 kbps is conducted. Furthermore, four slots composed of slots 5 through 8 are assigned to a user D having a transmission rate of 32 kbps and communication of 32 kbps is performed. According to the transmission rate at the time of a transmission request from each user, a base station or the like thus sets the number of slots in one frame to each user variably. As the result, it is possible to cope with the TDMA scheme by accommodating information bit streams having a plurality of bit rates mixedly and radio-transmitting them.
Furthermore, in the case where radio transmission is performed using a multi-carrier scheme called OFDM (Orthogonal Frequency Division Multiplex) scheme, for example, the configuration shown in FIG. 4 of a prior art is used as the transmission configuration. This configuration is a configuration applied to DAB (Digital Audio Broadcasting). An information bit stream obtained at a terminal 21 is subjected to processing such as coding in a coding unit 22, and thereafter mapped to transmission symbols in a symbol mapping unit 23. The transmission symbols are supplied to a mixing circuit 24, and multiplexed with other transmission data therein. As for the multiplexing conducted here, transmission data are simply coupled in series to generate a multiplexed symbol stream. For example, if symbols having 64 ksps per channel are multiplexed by 18 channels, then the transmission rate of the multiplexed symbol stream becomes 64 kspsxc3x9718=1152 ksps.
This multiplexed symbol stream is subjected to symbol rearrangement performed by frequency interleaving in a frequency conversion unit 25. As a result, symbols of respective channels are scattered. The rearranged symbol stream is converted into a multi-carrier signal arranged on the frequency axis by inverse Fourier transform processing in an inverse Fourier transform circuit (IFFT circuit) 26. The output of this IFFT circuit 26 is subjected to radio transmission processing in a radio processing unit 27, and radio-transmitted in a predetermined frequency band.
As for a configuration of the receiving side of this multi-carrier signal, a signal of a desired frequency band received by an antenna 31 is converted to a baseband signal in a reception processing unit 32 as shown in FIG. 5. Here, the baseband signal component of the multi-carrier signal is a signal having information pieces arranged on the frequency axis. Therefore, the multi-carrier signal is supplied to a fast Fourier transform circuit (FFT circuit) 33 and subjected to Fourier transform processing. As a result, subcarriers arranged on the frequency axis are extracted. At this time, symbols outputted by the Fourier transform processing become subcarriers of the whole received signal band.
The transformed signal of the subcarriers is supplied to a symbol selection unit 34. From symbol existence positions of a desired channel arranged by the frequency interleaving conducted on the transmission side, symbols are extracted. A symbol stream thus extracted is supplied to a bit extraction unit 35, where a coded bit stream is extracted. This coded bit stream is supplied to a decoding unit 36, and an information bit stream is obtained at an output terminal 37.
In this conventional OFDM scheme, multiplexing is conducted by assigning symbols of different channels to respective subcarriers. Therefore, the Fourier transform circuit (FFT circuit) possessed by the receiver performs transform processing on symbols corresponding to all multiplexed and transmitted channels, and channel selection is performed after the transform.
In the communication system of a cellular scheme to which the above described DS-CDMA scheme has been applied, data transmission of a variable rate is made possible by fixing the used frequency band and varying the spreading factor. By fixing the used frequency range, it will be possible to form a terminal device which provides variable bit rate service by using only a single high frequency circuit.
In the DS-CDMA scheme, however, the communication control scheme is very complicated. For example, in the case where it is applied to the cellular scheme, it is necessary to conduct hand off processing for switching over the base station, transmission power control for preventing any interference with other communication within the system, and the like with very high precision. Furthermore, in the DS-CDMA scheme, basically all channels share the same frequency band, and orthogonality of channels is not present. Therefore, the DS-CDMA scheme has a risk that the whole system does not function when there is even one terminal device in which transmission power control is not performed properly. It cannot be said that the system is suitable for performing complicated processing such as variable transmission rate.
Furthermore, in the case where variable transmission rate processing is applied to the DS-CDMA scheme, as regards the demodulation part, even a terminal device which makes communication at a low transmission rate of approximately several kbps needs to perform computational processing equivalent to that of a terminal device which conducts communication at the highest transmission rate possible in the system. As the result, the amount of computational processing in the terminal device significantly increases.
On the other hand, in the case where a variable transmission rate is implemented in a communication system to which the above described TDMA scheme is applied, the maximum transmission rate per channel is basically limited to [bit rate at the time when one slot is assigned]xc3x97[the number of TDMAS]. The-upper limit and the lower limit of the transmission rate are determined by the number of TDMAs. In the case where the variation range of the transmission rate is very large, such as approximately several kbps to approximately 100 kbps, therefore, it is substantially impossible to cope with the transmission rate that a user desires only by slot assignment. It is not impossible if a very large number of time slots are provided in one frame. From the viewpoint of communication control or the like, however, it is not practical.
Furthermore, in the case where multiplexing using a variable transmission rate is implemented in a communication system to which the above described conventional OFDM system is applied, multiplexing is conducted by assigning symbols of different channels to respective subcarriers. The Fourier transform circuit included in the receiver needs to perform transform processing on symbols corresponding to all multiplexed and transmitted channels. This results in a problem that a large amount of transform processing is required.
An object of the present invention is to make possible of information communication processing in a communication means such as a receiver with a minimum processing amount needed by itself when multiplexing channels through which communication is made at various transmission rates.
A communication method in accordance with a first invention, includes: setting a plurality of channels in a predetermined band; and conducting communication in each of the set channels by using a multi-carrier signal having transmission symbols distributed among a plurality of subcarriers, wherein the transmission symbols of each channel on a frequency axis are arranged at intervals of Nth power of 2 (where N is an arbitrary positive number) with respect to a reference frequency interval. In a transmission signal formed of a multi-carrier signal having multiplexed channels, therefore, transmission symbols of each channel are arranged at a predetermined frequency interval. On the transmission side, therefore, processing of forming a multiplexed transmission signal can be performed simply. In addition, it can easily be made to extract only signals of each channel and process for reception. The configuration of the reception side can be simplified. Furthermore, in the case of application to radio communication, wide-band communication is performed with broad subcarrier intervals and consequently it also becomes possible to obtain a frequency diversity effect.
In accordance with a second invention, the communication in the communication method according to the first invention is defined to be radio communication. Since wide-band communication is made with broad subcarrier intervals, therefore, it also becomes possible to obtain a frequency diversity effect.
In accordance with a third invention, a value of N in the communication method according to the first invention is variably set depending on a bit rate of transmission data. As a result, it will be easy to transmit data having different bit rates mixed together.
In accordance with a fourth invention, the communication method according to the first invention is applied to communication between a base station and a terminal device; one channel of down channels transmitted from the base station is secured as a pilot channel whereas remaining channels are used as traffic channels; in the base station, a known signal is transmitted by using said pilot channel; and in the terminal device, equalization processing of a transmission path of symbols received via said traffic channel is performed by using symbols received via the pilot channel, and synchronous detection of the symbols subjected to the equalization processing is performed. As a result, equalization processing of transmission signals can be performed easily and favorably.
In accordance with a fifth invention, a signal to be transmitted in the communication method according to the first invention is subjected to frequency hopping by taking a channel as a unit or by taking a frequency as a unit. As a result, multiplexed signals are spread and transmitted efficiently, and a favorable transmission state can be ensured.
In accordance with a sixth invention, a communication method includes: setting a plurality of channels in a predetermined band; conducting communication in each of the set channels by using a multi-carrier signal having transmission symbols distributed among a plurality of subcarriers; using subcarriers of every predetermined number of subcarriers, as subcarriers assigned to each channel; performing differential modulation between adjacent subcarriers among subcarriers assigned to each channel and thereafter performing transmission; and performing differential demodulation between adjacent subcarriers on a reception side. As a result, there is formed a multi-carrier signal using subcarriers of every predetermined number as channel arrangement. In addition, since the differential modulation is performed between adjacent subcarriers of each channel, it will be possible to perform transmission processing and reception processing by using only signals of each channel.
A seventh invention is such that, in the communication method according to the sixth invention, on a transmission side, differential modulation is performed between adjacent subcarriers on a frequency axis instead of performing differential modulation between adjacent subcarriers among subcarriers assigned to each channel; and on a reception side, differential demodulation is performed between adjacent subcarriers on a frequency axis instead of performing differential demodulation between adjacent subcarriers among subcarriers assigned to each channel. As a result, transmission processing becomes possible also by virtue of processing based on the arrangement of subcarriers on the frequency axis.
An eighth invention is a transmitter wherein: a multi-carrier signal having transmission symbols distributed among a plurality of subcarriers is generated; transmission symbols are arranged on a frequency axis in one channel of said multi-carrier signal, at intervals of Nth power of 2 (where N is an arbitrary positive number) with respect to a reference frequency interval; and the generated multi-carrier signal is transmitted as a predetermined channel among a plurality of channels set in a predetermined band. As a result, a multi-carrier signal having transmission symbols of each channel arranged at predetermined frequency intervals and having multiplexed channels is transmitted. Transmission symbols of each channel can be arranged by fixed processing. Such a transmission signal that allows easy multiplexing by means of simple processing can be formed.
In accordance with a ninth invention, a value of N in the transmitter according to the eighth invention is variably set depending on a bit rate of transmission data. As a result, it becomes easy to transmit data having different bit rates mixed together.
A tenth invention is such that, in the transmitter according to a eighth invention, transmission symbols of a plurality of channels are individually generated, and thereafter a multiplexed symbol sequence is generated by arranging symbols of respective channels symbol by symbol; multi-carrier signal generation processing is performed on the generated multiplexed symbol sequence collectively; and transmission processing is performed on a plurality of channels collectively. As a result, transmission processing of a plurality of channels can be performed collectively by using a simple configuration.
An eleventh invention is such that, in the transmitter according to the eighth invention, transmission symbols are generated, the generated transmission symbols being taken out as a signal on a time axis, and thereafter processing for convolving a frequency offset corresponding to a channel assigned to its own station being performed. As a result, processing for transmission at a desired frequency can be performed favorably by using a simple configuration.
A twelfth invention is such that, in the transmitter according to the eighth invention, a known signal is transmitted by using one channel among a plurality of transmission channels as a pilot channel, and transmission processing is performed by using remaining channels as traffic channels. As a result, transmission control can be performed favorably on the basis of the known signal transmitted via the pilot channel.
In accordance with a thirteenth invention, the transmitter according to the eighth invention includes frequency hopping means for performing frequency hopping on the generated multi-carrier signal by taking a channel as a unit or taking a predetermined frequency band as a unit. As a result, a frequency/interference diversity effect is obtained, resulting in more satisfactory transmission.
A receiver in accordance with a fourteenth invention receives a multi-carrier signal having transmission symbols distributed among a plurality of subcarriers, and performs reception processing on transmission symbols received in one channel at frequency intervals of Nth power of 2 (where N is an arbitrary positive number) with respect to a reference frequency interval. As a result, a multi-carrier signal having transmission symbols of each channel arranged at predetermined frequency intervals and having multiplexed channels can be received. By extracting transmission symbols at predetermined frequency interval and performing reception processing, a signal of a desired reception channel can be obtained. From a multiplexed and transmitted signal, a signal of a desired channel can be obtained easily.
A fifteenth invention is such that, in the receiver according to the fourteenth invention, among all symbols transmitted in a bandwidth used for communication, only symbols of a communication channel transmitted by a transmission side are extracted from a received signal; and the extracted symbols are supplied to a channel decoder for decoding. As a result, reception processing of only needed symbols can be performed efficiently.
A sixteenth invention is such that, in the receiver according to the fourteenth invention, sampling of a received signal is performed at a sample rate determined by a bandwidth of the received signal; by performing addition or subtraction on sampled symbols, a desired reception channel is selected to decrease the number of symbols outputted to a subsequent stage, and a required minimum sample rate determined by a maximum bit rate at time of reception is made; and reception processing is performed on received data having a number of symbols corresponding to the required minimum sample rate. As a result, received data having the number of symbols corresponding to the required minimum sample rate can be obtained efficiently.
In accordance with a seventeenth invention, the receiver according to the sixteenth invention includes a correction means for multiplying data of at least one reception channel by a sine wave offset correction signal when a plurality of reception channels has been selected. As a result, offset contained between data of each received channel can be removed simply.
An eighteenth invention is such that, in the receiver according to the sixteenth invention, reception processing means for performing reception processing on said received data has processing capability determined by a maximum bit rate, and when conducting communication at a bit rate lower than said maximum bit rate, only desired bits are extracted. As a result, an amount of data processed at the time of communication using a low bit rate can be reduced.
A nineteenth invention is such that, in the receiver according to the fourteenth invention, reception processing means of a pilot channel and reception processing means of traffic channels are provided, and the reception processing means of traffic channels performs traffic path equalization processing of a transmission path of received symbols of a traffic channel by using symbols of a known signal received by the reception processing means of the pilot channel. As a result, the equalization processing of the transmission path of the received symbols of a traffic channel can be performed favorably on the basis of the received signal of the pilot channel, which leads to good reception processing.
In accordance with a twentieth invention, the receiver according to the fourteenth invention includes frequency hopping means for performing frequency hopping on a received signal by taking a channel as a unit or taking a predetermined frequency band as a unit. As a result, reception processing of the transmission signal subjected to frequency hopping can be performed properly.